Not Applicable
Not Applicable
1. Field of the Invention
The present invention relates to hydraulically powered equipment, such as off-road construction and agricultural vehicles, and more particularly to apparatus for reducing bounce when a hydraulically driven member on the equipment is stopped suddenly.
2. Description of the Related Art
With reference to FIG. 1, a backhoe 10 is a common type of earth moving equipment that has a bucket 12 attached to the end of an arm 14 which in turn is coupled by a boom 15 to the frame of a tractor 18. A joint 16 enables the bucket, arm, and boom assembly 17 to pivot left and right with respect to the rear end of the tractor. A hydraulic cylinder 19 is attached on one side of the tractor 18 to the boom 15 and provides the drive force for the pivoting motion. For larger backhoes, a pair of hydraulic cylinders are attached on opposite sides of the tractor 18 to pivot the boom. Hydraulic fluid is supplied to the cylinder 19 through valves that are manipulated by the backhoe operator. This movement of the boom 15 is referred to as xe2x80x9cswingxe2x80x9d or xe2x80x9cslewxe2x80x9d.
As the boom swings, pressurized fluid is introduced into one chamber of the cylinder 19, referred to as the xe2x80x9cdriving chamberxe2x80x9d, and fluid is exhausted from the other cylinder chamber, referred to as the xe2x80x9cexhausting chamberxe2x80x9d. When the operator suddenly stops the boom swing, inertia causes the motion of the backhoe assembly 17 to continue in the direction of the swing. The amount of inertia is a function of the mass of the backhoe assembly 17 and any material carried in the bucket 12. This continued movement after the control valves have been shut compresses the hydraulic fluid in the previous exhausting chamber of the cylinder 19 and may produce a void, or cavitation, in the previous driving cylinder chamber. Anti cavitation valves typically are provided in the hydraulic system to overcome this latter problem.
Eventually the backhoe assembly 17 stops and starts moving in the opposite swing direction due to the relatively high pressure created in the previous exhausting chamber. This subsequent movement produces a reversal of the pressure conditions, wherein the previous driving chamber of the boom swing cylinder 19 becomes pressurized. As a result, the backhoe assembly 17 swing oscillates until inherent dampening provided by other forces eventually brings the assembly to a stop. This phenomenon is known either as xe2x80x9cswing bouncexe2x80x9d or xe2x80x9cswing wagxe2x80x9d and increases the time required to properly position the boom 15, thereby adversely affecting equipment productivity.
Various approaches have been utilized to minimize the swing bounce. For example, U.S. Pat. No. 4,757,685 employs a separate relief valve for each hydraulic line connected to the swing cylinder, which valves vent fluid to a tank line when excessive pressure occurs in that cylinder. Additional fluid is supplied from the supply line through makeup valves to minimize voids in the cylinder as the swing stops.
U.S. Pat. No. 5,025,626 describes a cushioned swing circuit which also has relief and make-up valves connected to the hydraulic lines for the boom swing cylinder. This circuit also incorporates a cushion valve which in an open position provides a fluid path between the cylinder hydraulic lines. That path includes a flow restriction orifice. The cushion valve is resiliently biased into the shut position by a spring and a mechanism opens the cushion valve for a predetermined time period when the pressure differential between the cylinder chambers exceeds a given threshold.
Both of the previous circuits required a number of relatively complex valves. Therefore, it is desirable to provide a more simplified mechanism for reducing swing bounce.
A hydraulic system includes a control valve assembly, which selectively couples a pump and a tank to a hydraulic actuator that drives a member on a machine. The system has a device which produces a command designating desired movement of the load. A sensor detects pressure in the hydraulic actuator.
A method is provided to reduce bounce of the member when it stops. A command is received from the device designating that movement of the member in a given direction is to stop. The signal from the sensor is employed to determine the rate at which the pressure in the hydraulic actuator changes. When the rate of change of the pressure is less than a defined threshold after receiving the command, pressure in the hydraulic actuator is relieved. For example the pressure is relieved by opening a control valve that is connected to the hydraulic actuator.
In one application, the present bounce reduction method is used on a machine in which the member is driven by a cylinder that has first and second chambers. It is a well-known practice that this type of installation includes first and second pressure relief valves that are respectively connected to the first and second cylinder chambers. Thus upon receiving the command, pressure in the second chamber is relieved by opening an associated control valve. Then a determination is made whether the first pressure relief valve is open due to excessive pressure in the first chamber. If the first pressure relief valve is found to be open, the bounce reduction method waits for that valve to close, and thereafter opens another control valve that relieves pressure remaining in the first chamber. Otherwise if the first pressure relief valve is found to be closed, the rate of pressure change in the first chamber is determined, and pressure in the first chamber is relieved by opening the other control valve when the rate of pressure change is less than a defined threshold.